This application is related to applicants concurrently filed application Attorney Docket No. PET 1839, entitled xe2x80x9cFlexible Process For Producing Base Stock And Distillates By Conversion-Hydroisomerization Using A Catalyst With Low Dispersion Followed By Catalytic Dewaxingxe2x80x9d, based on French Applications 99/05.496 filed Apr. 29, 1999 and 00/02.363 filed Feb. 24, 2000 and Attorney Docket No. PET 1840, entitled xe2x80x9cFlexible Process For Producing Base Stock And Middle Distillates By Conversion-Hydroisomerization Followed By Catalytic Dewaxingxe2x80x9d, based on French Applications 99/05.494 filed Apr. 29, 1999 and 00/02.364 filed Feb. 24, 2000.
The present invention relates to a catalyst used in processes for converting heavy feeds, in particular paraffin feeds containing reduced quantities of metals. This conversion is generally accompanied by hydroisomerization of n-paraffins.
When hydroconverting (particularly hydroisomerizing) feeds such as paraffin feeds from the Fischer-Tropsch process, or hydrocracking residues, it is of particular advantage to obtain very high value products such as base stock or middle distillates with a very good resistance to cold and a very good cetane index.
All catalysts currently used in hydroconversion and/or hydroisomerization are bifunctional, combining an acid function with a hydrogenating function. The acid function is provided by supports with large surface areas (in general 150 to 180 m2/g) with a superficial acidity, such as halogenated aluminas (chlorinated or fluorinated in particular), phosphorus-containing aluminas, combinations of oxides of boron and aluminium, silica-aluminas and aluminosilicates. The hydrogenating function is provided either by one or more metals from group VIII of the periodic table such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, or by combining at least one group VI metal such as chromium, molybdenum or tungsten and at least one group VIII metal.
The balance between the two functions, acid and hydrogenating, is the fundamental parameter which governs the activity and selectivity of the catalyst. A weak acid function and a strong hydrogenating function produces catalysts with low activity which are selective towards isomerization while a strong acid function and a weak hydrogenating function produces catalysts which are highly active and selective towards cracking. A third possibility is to use a strong acid function and a strong hydrogenating function to obtain a highly active catalyst which is also highly selective towards isomerization. Thus by careful choice of each of the functions, it is possible to adjust the activity/selectivity properties of the catalyst.
A number of processes and catalysts exist.
U.S. Pat. No. 5,834,522 and International patent application WO-A-95/26819 describe an amorphous catalyst based on a noble metal and silica-alumina with precise physico-chemical characteristics, including a noble metal dispersion in the range 20% to 100%.
In that process, and generally in all catalytic processes, it is known that to improve the performance of the catalyst, care must be taken that, among other factors, the dispersion of the noble metal is as high as possible. Thus during generation, for example, the operating conditions are precisely set so as to avoid the formation of agglomerates of metal and/or to re-disperse the agglomerated metal.
In complete contrast to that commonly accepted fact, during the course of research on the metallic phase it has been discovered that a low dispersion of the noble metal, advantageously associated with a particular distribution of metal particles, results in catalysts which are even more selective for isomerization.
More precisely, the invention provides a catalyst containing at least one noble metal deposited on a support, the dispersion of noble metal being less than 20%.
Preferably, the fraction of noble metal particles less than 2 nm in size represents at most 2% by weight of the noble metal deposited on the catalyst.
Advantageously, the size of at least 70% (preferably at least 80%, more preferably at least 90%) of the noble metal particles is over 4 nm (number %).
The support is advantageously an amorphous acidic support and contains no molecular sieve, the catalyst thus contains no molecular sieve.
The acidic support can be selected from the group formed by a silica alumina, boron oxide, a zirconia used alone or as a mixture of the two or with a matrix (for example non acidic).
The acidic support is generally selected from the group formed by a silica-alumina, a halogenated alumina (preferably fluorinated), an alumina doped with silicon (deposited silicon), an alumina-titanium oxide mixture, a sulphated zirconia, a zirconia doped with tungsten, and mixtures thereof or with at least one amorphous matrix selected from the group formed by alumina, titanium oxide, silica, boron oxide, magnesia, zirconia and clay, for example.
Preferred supports are amorphous silica-alumina and silica-alumina-titanium oxide (amorphous).
The acidity measurement is well known to the skilled person. It can, for example, be made by temperature programmed desorption (TPD) with ammonia, by infrared measurement of absorbed molecules (pyridine, CO . . . ), by a catalytic cracking test or by hydroconverting a model molecule.
A preferred catalyst of the invention comprises (and is preferably essentially constituted by) 0.05% to 10% by weight of at least one noble metal from group VIII deposited on an amorphous silica-alumina support.